INTRODUCTION

ABSTRACT

Concern for the environment has led to legislation that is driving the use of chemicals in the off shore North Sea oil industry. The increased awareness and scrutiny of the impact of chemical discharge on the environment is likely to ensure adoption of these, or similar policies, on a global basis. The need for low toxicity, biodegradable and hydrothermally stable scale inhibitors is increasing and greener chemistries are being sought to meet these challenges. This paper describes a barium sulphate and calcium carbonate scale inhibitor that meets low toxicity requirements and the present Norwegian 'Y1' criteria for biodegradation. Dynamic scale loop tests are used to evaluate the inhibitor compared to a commercial 'green' inhibitor for carbonate and sulphate scales. Hydrothermal studies performed under differing pH regimens demonstrate stability to 180°C and a marginal drop in performance at 200°C thereby showing good potential for squeeze treatment under HPHT conditions in addition to topside application.

Scale control inhibitors play a major role in flow assurance. They have been used for many years in both squeeze treatment applications and also in the treatment of topside facilities. Once their job is complete they were discharged into the sea. In recent years there has become a heightened concern over potential damage to the environment from these discharged chemicals and the impact they may have on marine ecosystems. Re-injection of produced water has gone some way in mitigating the risk to the environment, however, there is still a need for greener chemistries to reduce the impact further.

International guidelines and regulations have/are being put in place in various parts of the world1 but there are differences of opinion, even between countries using the same regulations2. The challenge for any green inhibitor is not only to meet or exceed these regulations but also to be able to inhibit mixed scale, both downhole and at the surface separations facilities3,4. A global harmonised system needs to be put in place. If global agreement cannot be reached then it should be incumbent on operators, service providers and chemical manufacturers to voluntarily ensure that the best chemicals and practices be used to protect the environment.

In countries where no regulations are in place, self regulation by the deployment of chemistries that meet the most stringent regulations that have been introduced in other parts of the world should be adhered to. The selection of chemistries should be based on the most efficient inhibitors to ensure flow, whilst being benign to the environment, and still give efficient.

There appears to be a fundamental mismatch in requiring efficient biodegradation and an ability to withstand the higher temperatures being encountered downhole. Existing green inhibitors have not found wide acceptance for deployment as squeeze chemicals due to their inherent hydrolytic and thermal stability issues. While the polycarboxylic acid (PCA) inhibitor presented here has performed well in the OECD 306 test it has also shown a resistance to hydrothermal degradation over a range of pH and temperatures up to 200°C.

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